Apple, Google, eBay, and Priceline, meanwhile, are trading at only 15X next year's estimated earnings. (And that's before Priceline's post-earnings collapse, which will bring the multiple down modestly).

Analysts' projected estimates for Facebook appear to be a fair guess at what the company will earn next year, not low-balled numbers that the company will almost certainly beat

All of these attributes warrant less of a premium (and, possibly, no premium at all).

Now, deciding on a "fair" earnings multiple for a stock is a subjective exercise, and there are a wide range of reasonable and defensible arguments for all stocks, including Facebook. The one rule of thumb is that when investors are getting more exuberant and excited about a company's earnings prospects, multiples generally increase. When investors are getting more gloomy and cautious, multiples decrease.

Right now, investors are still getting more gloomy and depressed about Facebook, in part because of the "meh" quarter the company just reported. If Facebook were to post surprisingly strong results in its next quarter, however, the market's mood would change quickly, and Facebook's multiple would likely increase.

Further complicating matters, it's also a subjective exercise to choose which companies to compare Facebook's multiple to.

Many investors who own Facebook (and want it to go up), for example, reject the idea that Facebook should be compared to Apple, Google, Priceline, and eBay. Instead, they say, it should be compared to LinkedIn and Amazon, two other hot tech companies that have much higher earnings multiples.

Amazon, for example, is trading at nearly 100X next year's estimated earnings, and LinkedIn is trading at nearly 90X. Next to those multiples, Facebook's 30X doesn't look so extreme.

However, there are several reasons why comparing Facebook to LinkedIn and Amazon may be wishful thinking:

The profit margins for both LinkedIn and Amazon are currently depressed by heavy investment and are expected to rise radically in future years. This is a subtle but very important difference between Facebook and Amazon/LinkedIn. Facebook had an extraordinary 50% operating margin last year, which suggests its margin has only one way to go--down. This means that earnings can grow no faster than revenue and may grow more slowly than revenue. Amazon/LinkedIn, meanwhile, have margins that are currently depressed by heavy investment and are expected to expand rapidly in the next couple of years, driving earnings growth that is much faster than revenue.

The expected earnings growth for both LinkedIn and Amazon is much faster than Facebook. LinkedIn's revenue is expected to grow ~80% this year, versus about 40% for Facebook, and LinkedIn's margin is expected to rise. Amazon's revenue is still growing 30% per year, and its margin is expected to nearly triple over the next couple of years. This will drive huge earnings growth at both companies. Facebook's earnings growth, meanwhile, is expected to be a modest 30%. All else being equal, faster earnings growth warrants a bigger multiple.

There's less uncertainty with LinkedIn and Amazon. Amazon is a very established business with a well-understood business model. LinkedIn, meanwhile, has several powerful, diversified revenue streams. Facebook, on the other hand, still has only one main revenue stream, and there is a lot of controversy about the sustainability and effectiveness of its product.

LinkedIn and Amazon are likely to be less affected by the transition to mobile as Facebook is. LinkedIn's main businesses are professional subscriptions, which are helped, not hurt, by the mobile transition (because they make the service more accessible). Same for Amazon. Facebook, meanwhile, has to deal with smaller screen sizes and lower tolerance for ads, suggesting that the mobile transition could hurt the company.

LinkedIn's estimates, at least, are considered conservative, suggesting that the stock's multiple is not actually as high as it appears. Investors think LinkedIn will continue to blow away its earnings estimates each quarter. By the time we get to mid-2013, therefore, the "earnings" analysts are using to value the stock should be a lot higher. Facebook's meanwhile, may be the same (or lower).

So you see, there are many reasons to think that Facebook does not deserve the super-high multiples enjoyed by LinkedIn and Amazon.

Also, if LinkedIn or Amazon were to stumble, their stocks (and multiples) would get crushed. Either stock could trade at half its current multiple or lower and still seem "expensive," so there's plenty of downside in both. For now, the market expects that there is huge potential earnings growth "embedded" in these companies, so it's willing to pay up. But arguing that Facebook's multiple should be closer to the multiples of stocks that have screamingly high multiples that might get crushed is not exactly conservatism.

So, what is the right multiple for Facebook?

There's no right multiple. Valuations are based on the future and no one knows for sure what the future holds. Ultimately, stocks are worth what other people will pay for them, and valuation is always a subjective exercise.

That said, an objective analysis of the situation suggests--at least to me--that Facebook's multiple is still uncomfortably high.

This analysis suggested that Facebook is now, finally, getting toward the middle of that range.

However...

Facebook's earnings outlook has actually deteriorated modestly since the IPO, which is one of the reasons the stock has crashed.

Wall Street analysts (and I) expected Facebook to handily beat the Q2 guidance it gave analysts during the IPO roadshow, which would have suggested that the estimates for 2013 earnings were too conservative and would rise.

But Facebook only met its Q2 guidance. So the 2013 estimates did not go up.

And now, instead of the $0.80-$1.00 that most analysts quietly expected Facebook to earn next year, it looks as though the current estimate of ~$0.65 is reasonable (or even aggressive).

So that changes the "fair" valuation modestly.

Applying the same "fair" multiple range of 20X-30X to projected 2013 earnings of $0.65 instead of $0.80, I get a "fair value" range for Facebook of $13-$20.

This doesn't mean that Facebook will ever trade in that range (though it's getting close). It only means that $13-$20 seems like the level at which Facebook's valuation seems "fair" as opposed to "expensive."

Again, this is all subjective. If Facebook gave the market something to get excited about, the stock could reverse its decline and trade at a super-premium multiple for several years. On the other hand, given the upcoming lock-up releases and the potential for Facebook's outlook to further deteriorate, the stock's multiple could continue to compress.

In any event... the logic above is why you're not going to hear me say that Facebook is "cheap" at $20. It isn't.